1. Field of the Invention
The present invention relates to a method of manufacturing an external force detection sensor, such as an angular velocity sensor and an acceleration sensor.
2. Description of the Related Art
FIG. 6A illustrates a top plan view of an angular velocity sensor which is an external force detection sensor, and FIG. 6B illustrates a section taken along the line A—A indicated in FIG. 6A. A sensor element 1 to constitute the angular velocity sensor illustrated in FIG. 6A and FIG. 6B is of predetermined shape in which an element substrate 3 (e.g., a semi-conductor substrate such as a single crystal silicon substrate) to be joined with, for example, a glass support substrate 2 is dry-etched.
As illustrated in FIG. 6A and FIG. 6B, an oscillating body 5 is arranged in a floating condition from the above-described support substrate 2 above a surface 2a which is a surface in the direction of the X-Y plane of the above-described support substrate 2. In the oscillating body 5, a weight 5b is provided inside a frame body 5a. A plurality of (four in an example in FIGS. 6A and 6B) fixed parts 6 are arranged on the support substrate 2 in a fixed manner with intervals from each other, and the oscillating body 5 is supported in an oscillating manner in the X-direction and the Y-direction by each fixed part 6 through each L-shaped support beam (beam) 7.
Comb-toothed movable electrodes 10 (10a, 10b) are formed on right and left sides of the oscillating body 5 as viewed in FIG. 6A outwardly in each direction of the X-axis, and fixed comb-toothed electrodes 11 (11a, 11b) to be engaged with the movable electrodes 10 through clearance are extended inward of fixed parts 12 in each direction of the X-axis.
An electrically conductive pattern not illustrated is connected to the comb-toothed electrodes 11a, 11b, and the voltage is applied to the fixed comb-toothed electrodes 11a, 11b from each external side through the electrically conductive pattern. For example, when AC voltages different in phase by 180° are applied to the fixed comb-toothed electrodes 11a, 11b through the electrically conductive pattern-with the movable electrodes 10a, 10b in the condition of a specified fixed voltage (e.g., zero V), electrostatic forces opposite in direction to each other are generated between the movable electrode 10a and the fixed comb-toothed electrode 11a, and between the movable electrode 10b and the fixed comb-toothed electrode 11b, and the oscillating body 5 is excitation-oscillated in the X-direction by the electrostatic forces.
Movable electrodes 13 (13a, 13b) are extended on upper and lower sides of the oscillating body 5 as viewed in FIG. 6A in each longitudinal direction, i.e., each direction of the Y-axis, and fixed electrodes 14 (14a, 14b) opposite to the movable electrodes 13 through a clearance are extended inward of a fixed part 15 in the longitudinal direction.
In the angular velocity sensor (external force detection sensor) of the above-described constitution, a Coriolis force is generated in the Y-direction when the external force detection sensor is rotated about a Z-axis orthogonal to the direction of the X-Y plane in a condition where the oscillating body 5 is excitation-oscillated in the X-direction as described above. The Coriolis force is applied to the oscillating body 5, and the oscillating body 5 is oscillated in the direction of the Coriolis force. The clearance between the above-described movable electrodes 13 and fixed electrodes 14 is changed by the oscillation in the Y-direction of the oscillating body 5 attributable to the Coriolis force, and the electrostatic capacity between the movable electrodes 13 and the fixed electrodes 14 is changed. The magnitude of the angular velocity of the rotation can be detected by detecting the electrical signal corresponding to the magnitude of the amplitude of the oscillation in the Y-direction of the oscillating body 5 generated by the above-described Coriolis force making use of the change in electrostatic capacity. Thus, the sensor element 1 of the angular velocity sensor illustrated in FIGS. 6A and 6B forms a movable element having a movable part such as the oscillating body 5 and a support beam 7.
An example of a conventional method of manufacturing the angular velocity sensor illustrated in FIGS. 6A and 6B is briefly described referring to FIG. 7A to FIG. 7E. For example, as illustrated in FIG. 7A, a recess 16 is formed on a back surface 3b of the element substrate 3 by a dry etching technology such as RIE (Reactive Ion Etching) to form, for example, a membrane (diaphragm) 17 of 60-70 μm in thickness d.
As illustrated in FIG. 7B, an etching stop layer 18 comprised of silicon oxide is formed on a top surface 16a of the above-described recessed part 16 using a film forming technology such as CVD (Chemical Vapor Deposition).
As illustrated in FIG. 7C, the support substrate 2 is arranged on the back surface 3b side of the above-described element substrate 3, the support substrate 2 and the element substrate 3 are heated to high temperature, and a high voltage is applied thereto to anode-join the support substrate 2 with the element substrate 3.
After that, the membrane 17 of the above-described support substrate 2 is machined making use of a photolithography method and RIE to form a plurality of through holes 20 reaching from the surface 3a of the element substrate 3 to the above-described etching stop layer 18 as illustrated in FIG. 7D, and the sensor element 1 is formed by forming the oscillating body 5, the support beam 7, the movable electrode 10, the fixed comb-toothed electrode 11, the movable electrode 13, the fixed electrode 14, etc. by a plurality of these through holes 20. In this specification and the claims, the dry etching technology to form the through holes to be passed from the face side to the back surface side of the substrate is referred to as “through-hole dry etching”.
As described above, after the sensor element 1 is formed, the etching stop layer 18 is removed by a wet etching process using a buffer hydrofluoric acid aqueous solution as illustrated in FIG. 7E. The angular velocity sensor as illustrated in FIGS. 6A and 6B can thus be manufactured.
The etching stop layer 18 to be formed in manufacturing the external force detection sensor such as the angular velocity sensor has to be conventionally formed of an insulating material such as silicon oxide from the viewpoint of facilitation of forming a layer and simplification of a manufacturing process of the external force detection sensor. However, the inventor noticed that a notch (a profile distortion) is formed on a lower part side (i.e., a side on which the etching stop layer 18 is formed) of a side wall surface of the through holes 20 as illustrated in FIG. 7E since the etching stop layer 18 is formed of the insulating material as described above.
This may be considered attributable to the following reason. For example, when the element substrate 3 is machined by through-hole dry etching to form the sensor element 1, the etching removal is achieved faster in a hole larger in etching removal area such as a through hole 20a between the frame body 5a and the weight 5b of the oscillating body 5 illustrated in FIG. 6A than in a hole smaller in etching removal area such as a through hole (an etching groove) 20a between the movable electrode 10 and the fixed comb-toothed electrode 11 by the micro-loading effect.
The time required to achieve the etching removal up to the etching stop layer 18 and complete the forming of the through holes 20 after the through-hole dry etching is started is different for each through hole 20 by the difference in the above-described etching removal area. Since the above-described through-hole dry etching is continuously performed until the forming of all through holes 20 is completed, some through holes 20 which are continuously exposed to an etching gas though the etching removal is completed are generated (hereinafter, these through holes are referred to as the “over etched through holes”).
The etching gas continuously enters such over etched through holes 20 during the over-etching, and the etching stop layer 18 at a bottom part of these over etched through holes 20 is charged positive by the collision of the positive ion in the etching gas.
When the etching is continued even after the etching stop layer 18 is charged positive and the etching gas continuously enters inside the over etched through holes 20, the positive ion in the etching gas is advanced straight toward the etching stop layer 18 inside the through holes 20, but repulsed by the positive charge of the above-described etching stop layer 18 immediately before the positive ion reaches the etching stop layer 18. In addition, the side wall surface of the over etched through holes 20 is charged negative by the collision of the electron in the etching gas, and thus, the above-described positive ion is attracted to the side wall surface of the through holes 20 immediately before it reaches the etching stop layer 18, and the course of the positive ion is largely curved. As a result, the positive ion in the etching gas collides with the bottom we side (the side on which the etching stop layer 18 is formed) of the side wall surface of the through hole 20 to form the notch n as illustrated in FIG. 7E.
Since the etching stop layer 18 is formed of the insulating material, it is found that the following problem can occur. When no through holes 20 are completed yet while the through hole dry etching is performed to form the through holes 20, as shown in FIG. 8A, the heat generated in the side wall surface of the through holes 20 (as shown by the arrows in FIG. 8A) is diffused into the membrane 17, and the area to be dry-etched of the membrane 17 or the like is heated to the same temperature almost over the whole area.
However, when through holes 20A are generated during the over etching as illustrated in FIG. 8B, the temperature of a part between the over etched through holes 20A (for example, a hole indicated by the reference numeral 21 in FIG. 8B) rises. That is, when the electrons in the etching gas collide with the side wall surface of the through hole 20A during the over etching to generate heat, the hole 21 is thermally independent from other areas since the etching stop layer 18 is formed of the insulating material and its heat conductivity is very inferior, the heat is stored in the side wall surface of the hole 21, and the temperature of the hole 21 rises higher than the other areas. Thus, the hole 21 becomes easier to etch than the other areas, the etching removal is excessively achieved as indicated by a solid line while the true etching removal should be originally achieved to the dimension as indicated by a broken line in FIG. 8C, resulting in a part not being formed to the designed dimension because of the excessive etching.
As described above, the inventor noticed that the notch n is formed on the etching stop layer 18 side of the side wall surface of the through holes 20, excessive etching is generated, and the sensor element 1 can not be formed to the designed dimension with excellent accuracy since the etching stop layer 18 has been formed of the insulating material in a conventional practice. For example, since the sensor element 1 can not thus be formed with excellent dimensional accuracy, stable output sensitivity of the external force detection sensor can not be obtained.